Electron optical system and method



June 8, 1948. J. l-uLLn-:Rr

ELECTRON OPTICAL SYSTEM AND METHOD Filed July .'50, 1945 INVENTOR. Jwzcjler wWu 47% June 8, 1948. J. HlLLlER ELECTRONy OPTICAL SYSTEM ANDMETHOD Fild July so, 1945 2 Sheets-Shea?I 2 R. mr y m. w. j W. M l u,lmllllm IHA! w W ,M W y f f L B M ya! j rnAlllll .j f m V W A W w n W wM w m f f f P Patented June 8, 1948 'ELECTRON OPTICAL SYSTEM AND METHODJames Hillier, Granbury, N. J., assigner Ato Radio lCorporation ofAmerica, a corporation of Dela- Application July 30, 1945, Serial No.607,849

(Cl. Z50-49.5)

Claims. 1

This invention relates generally to electron optical systems and methodsand more particularly to an improved method of and means for exploring amicrospecimen with -anelectron beam by selectively varying the focus of`the electron beam with respect to the specimen to provide a shadowimage, an electrondiffrac-tion pattern, or a combination of said kimageand said pattern on an electron responsive screen.

Immediately following the initial use -o'f the electron diffractioncamera as a means of demonstrating the wave'properties of an electronbeam, its further use as a means of electron analysis was recognizedthrough analogy to X-ray diffraction methods. Early electron diffractioncameras followed the same basic technique as X-ray diffraction cameras,consisting essentially of the use of an electron source, aco1limating'system arranged to produce a narrow electron beam, aspecimen subjected to said collimating beam, and a` fluorescent screenincluding means `for permanently recording the diffraction patterns. Itwas later found that the addition of a'focusing lens on either side ofthe specimen greatly improved the sharpness of the diffraction patterns.

A focused diffraction camera attachment for a conventional electronmicroscope was disclosed and claimed 'by James Hillier and Richard F.Bakerin their copending U. S. application Serial No. 441,142, filed onApril 30, 1942, Patent No. 2,403,529, granted July 9, 1946. The devicedisclosed therein comprised a separate specimen chamber including acollimating aperture and a projection lens for focusing the diffractionpat tern upon the fluorescent screen of the electron microscope. Howeverthe use of the diffraction camera attachment necessitated the transferof the specimen from the microscope specimen chamber to the. diffractioncamera specimen -chamber for obtaining electron diffraction patterns.

Another type of diffraction camera is disclosed and claimed in thecopending application of Perry C. Smith, Serial No. 585,561, filed March29., 19,45., PatentNo. 2,422,807, granted June 24, 1947, in which theelectron source is focused to a relativelysmall spot on the specimen inorder that a diffraction pattern of a minute area of said specimen mightbe obtained,

None of the previously disclosed electron diffraction cameras providedfor careful, correlation of the morphology of the specimen as observedinthe light or electron microscope with the nature of the specimenmaterial as shown by an electron diffraction pattern, of the sameportion 2 i of the specimen. Such correlation Lis extremely valuable forproper analysis of 'the portion of the speci-men under observation.

The instant invention comprises 'an improved electron diffraction cameraproviding a sharply focused and magnified diffraction pattern at thefluorescent screen and includesmea-ns for selectively providing ashadowV image of the same portion of the specimen at the fluorescentscreen Without the lnecessity of changing the position of the specimenin 'the system.r Further-more, means are provided for graduallydissolving from a sharply focused electron diffraction pattern, througha combination of a diffraction pattern and a shadow image, to a highlymagnified shadow image of the specimen. The improved system utilizes vanelectron source and an. electron focusing lens and limiting aperture forirradiating a selected minute area of the `specimen under observation. Asecond electron lens'having variable magnification and focus is disposedadjacent to the specimen. When the ysecond lens is focused to a pointclosely adjacent to the specimen, a highlymagnied shadow image isprovided upon the fluorescent screen. As a focal point of the lens ismoved toward the fluorescent screen, a combination shadow image andelectron diffraction pattern is obtained on the screen for selectedminute specimen areas. When the second lens is focused to a point on thefluorescent screen, a highly magnified, sharply focused diffractionpattern of the selected specimen area is provided on the screen. Thefocus of the second lens may be Varied 1in discrete steps or it may becontinuously adjustable for dissolving a shadow image to an electrondiffraction pattern of high magnification which is sharply focused onthe screen. f

Previously known diffraction cameras have had insuliicient resolvingpower to indicate diffraction ring widths of anybut a few of the mostcommon test specimens such as,` for example, evaporated films ofaluminum, silver or gold. However most other materials having naturalring widths as much as two orders of magnitude smaller than aluminum,silver or gold could not be satisfactorily observed. The sharply focuseddiffraction patterns provided .by the instant system thus greatly extendthe useful range of electron diffraction technique. vAlso the ability toobserve the shadow image of the portion of the specimen underobservation greatly facilitates electron analysis of specimen material.y

It is difficult to assign a satisfactory value of resolving power to anelectron diffraction camera If, as is usually the case in electrondiffraction patterns, the diameter of the ring is much greater than theelectron spot size, the radial intensity distribution in a ring obtainedby uniformly superimposing such spots is Ir=lc[(d/2)2:c2l1/2 where 1c isan instrumental constant, and :c is the radial distance from the centerof the distribu- 4 magnification of the system is maintained above10,0009;

Among the objects of the inventionare to provide an improved method ofand means for exploring a microspecimen by means of a focused electronbeam. Another object is to provide improved methods of and means forselectively or simultaneously providing electron shadow images orelectron diffraction patterns of a selected area of a microspeci-men. Anadditional object of the invention is to provide a method of andmeansfor employing a variable focus electron lens system for irradiating amicrospecimen to selectively provide a magnified sharply focuseddiffraction pattern or a magnified shadow image of a selected area ofthe specimen. A further object of the invention is to provide animproved method of y and means for continuously dissolving from a tion.Thus the half value width (1.73 'zZ/2) of this distribution may beemployed as a measure of the resolving power providing that the rings tobe resolved are of equal intensity. v

Using Braggs law and the geometry of the instrument, and assuming thatthe angle of deflection is small, the expression for the resolving powerof an electron diffraction camera is:

where dan, is the lattice spacing, l is the distance between thespecimen and the photographic plate, and }\=(150V)1/2 is the wavelengthassociated with a beam of V electron volts energy.

In this expression:

where dk is the diameter of the electron source, m is the magnification5of the lens system, ai is the distance between the final lens and thephotographic plate, fz is the focal length of the final lens, Ic is thespherical aberration constant of the nal lens (f2=f2o(1i-7cr2) and 1' isthe radius of the aperture used in the final lens. 'Ihe added term Srepresents the change in spot size which arises as a result of thediffusing effect of any accumulated charge on the specimen.

In early types of single lens diffraction cameras it is found that thelimiting resolving power is in the range of 1/70 to 1/170 for cZhk1=1.5Angstrom units and }\=.055 Angstrom units. However for the improveddouble lens'diffraction system described herein, the resolving `powermay be made to have a limiting value of `better than 1/10000 althoughthis value is seldom attained in practice due to the appreciable effectsof the specimen on the focusing of the spot.

The relatively high resolving power of the double lens system lendsitself well to inclusion of the shadow microscope feature describedheretofore. While the electron microscope images obtained in this mannerare not of the high quality of those obtained with conventional electronmicroscopes, they are quite satisfactory for many purposes.

The resolving power of the instrument operated as a shadow type electronmicroscope is given by the expression:

Where mz is the adjusted value of the magnification of the second lensand mi is the magnification (electronic) of the image of the specimen.In a practical case the focal length of the second lens is reduced fromcm. to 0.2 cm. (making m2=0.023) which leads to a resolving power of theorder of 200 A., providing that the electronic magnified shadow image toa magnified sharply focused diffraction pattern of a microspecimen. Anadditional object is to provide' an improved methodof and means forsimultaneously'producing a magnied shadow image andY magnified focuseddiffraction pattern of a microspecimen. Another object is to provide animproved method of and means for providing simultaneously or selectivelymagnified shadow images or magnified focusedl electron diffractionpatterns of a microspecimen in a fixed position within the resolvingapparatus. v

The invention will be described in further detail by reference to theaccompanying drawing of which Figure 1 is a schematic diagram' of apreferred embodiment thereof, and Figure 2 is a family of ray diagramsillustrating the focusing of the electron beam for providing shadowimages, diffraction patterns, or a combination of images and patterns.Similar reference characters are applied to similar elements throughoutvthe drawing.

Referring to the drawing, an intense, relatively small electron sourcerI, of the type ordinarily employed in conventional electron microscopes,is focused by means of a first electromagnetic lens 3 having pole pieces5 and 1 and a limiting aperture device 9 to a focal point II. The firstelectromagnetic lens 3 includes a solenoid winding I3 which is energizedfrom a regulated lensfpowerV supply I5 through a variable seriesresistor I 'I which may be adjusted to control the lens magnincation andfocus. f

The paraxial portion of the focused electron beam derived from the firstlens 3 is again focused by means of a second electron lens I9. Thesecond electron lens I9 includes pole piecesZI, 23, and an extremelyfine limiting aperture device 25. The eld of the second lens I 9 isprovided by means lof a second solenoid winding 21 which also isenergized from the regulated lens power supply I5 through a secondvariable series resistor 29. A three-position switch 3| connected in thesecond lens circuit includes a movable switch arm 33 `which may beoperated to select any one ofv three fixed contacts 35, 31, and 39. Thefixed contact 35 is connected to a movable'contact on the secondresistor 29, whilel the fixed contacts 31 and 33 are connected torelatively fixed positions on said second resistor. Thus, by actuationof the switch 3 I, either two fixed magnifications and focal lengths maybe provide-d for the second lens I3, or the movable contact may beadjusted on the'second resistor 29 to provide continuously variablemagnification and focal length for said lens.

The miropemen t0 be observed is located close to the second'lens i9andpreferably intermediate the' liis and' a ur'sceiit' screen 4l. Thefluorescent screen 4I maybe hinged tofpermit impingement of the electronbeam-upon a photographic plate 41"' disposedl iinmediatlely-v be'- hifndthe screen. vThe portion. ofthe rayl-` diagram in lines 4'3"indicatsfthemann in which"-a shadowmirnage -of the specimenniaybe formed at" thefluorescent' screen 4|. The size and magnification of the shadow imagewill be determined by the distance from the specimen to the focal point45 of the beam, and by the distance from the specimen to the fluorescentscreen.

When the second lens I9 is adjusted to focus the electron beam to apoint 41 on the fluorescent screen 4I, a magnified, sharply-focused,diffraction pattern of the selected specimen area will be formed on thefluorescent screen, as indicated by means of the solid line diffractedray y'49 which is representative of a single focused diffraction ring.

Referring to Figure 2, it is seen that if the second lens power isadjusted to provide a focal point for the electron beam closely adjacentto the specimen (on either side of the specimen), a greatly magnifiedmicroscope shadow image will be provided at the fluorescent screen orphotographic plate 4l', as indicated in ray diagram (a). Ray diagram (b)illustrates the manner in which a diffraction ring is sharply focused atthe point '5I on the fluorescent screen or photographic plate 4| whenthe second lens is adjusted to focus the electron beam at the point 53at the center of said screen.

Ray -diagram (c) shows an intermediate focal length for the electronbeam wherein it is focused at a point 55 intermediate the specimen andthe photographic plate 4I'. With this adjustment of the power of thesecond lens I9, a shadow image of lower magnification and a diffractionpattern also of lower magnification and lacking sharp focus will beprovided on the photographic plate 4I. The magnification of the shadowimage and diffraction pattern as well as the focus of the diffractionpattern will vary continuously as the focal point 55 of the electronbeam is varied intermediate the specimen and photographic plate l4|'.

Thus the invention described `comprises an improved method of and meansfor providing selectively lor simultaneously electron shadow images orelectron diffraction patterns of a specimen in a fixed position in theresolving apparatus. Means are provided for varying the magnification ofa single lens for discretely or continuously dissolving from a highlymagnified shadow image to a finely-focused diffraction pattern of thespecimen.

I claim as my invention:

1. The method of exploring a specimen with an electron beam constitutedof focused electron rays and producing indications of said .explorationupon an electron-sensitive screen, said method comprising generatingsaid electron rays, and selectively varying the focus of said rays withrespect to said specimen and said screen for selectively providing ashadow image, an electron diffraction pattern or a combination of saidimage and said pattern of said specimen `on said screen.

2. The method of exploring a specimen with an electron beam constitutedof focused electron rays to selectively obtain electron shadow imagesand electron diffraction patterns of selected portions of said specimen,said method comprising lli generating sald electron" rays, selectively;focusmen tojprovideA asliadowiimage: of' said specimen,

andchangingv thefo-cusofi said rays-for selectivelyelectron'iira'diati'ng' aI selected area; of said specimen to" provideanelectron diffraction pattern of said selected specimen area.

3, The method of exploring a specimen with anl electron' seamconstituted or focused i electron rays, said method comprisinggenerating.: said4 electron rays, selectively focusing said rays to afocal point adjacent to said specimen and intermediate said specimen andthe source of said rays to provide a shadow image of said specimen,changing the focus of said rays to provide simultaneously a shadow imageand an electron diffraction pattern of said specimen, and furtherchanging the focus of said rays to provide a focused electrondiffraction pattern of said specimen.

4. The method of exploring a specimen with lan electron beam constitutedof focused electron rays and producing indications of said explorationupon an electron-responsive element disposed on the side of saidspecimen remote from the source of said rays, said method comprisinggenerating said electron rays, selectively focusing said rays to a focalpoint adjacent said specimen and intermediate said specimen and saidsource to provide an enlarged shadow image of said specimen at saidelement, changing the focus Iof said rays to a point intermediate saidspecimen and said element to provide a simultaneous shadow image andelectron diffraction pattern of said specimen at said element, andfurther changing the focus of said rays to the plane of said element toprovide a focused electron diffraction pattern of said specimen at saidelement.

5. The method defined in claim 4 wherein said focusing of said rays ischanged in discrete steps.

6. The method defined in claim 4 wherein said focusing of said rays ischanged continuously between predetermined focal limits.

7. An electron optical device including a source of electron rays, anelectron-sensitive screen, means for supporting a specimen in the pathof said rays intermediate said source and said screen, electron focusingmeans for said rays disposed intermediate said source and said specimensupporting means, and means operableupon said focusing means forselectively varying the focus of said rays from a point adjacent to saidspecimen and intermediate said specimen and said source to a point onsaid screen for selectively deriving an enlarged shadow image, anelectron diffraction pattern, and a combined shadow image anddiffraction pattern of said specimen on said screen.

8. An electron optical device including a source of electron rays, anelectron-sensitive screen, means for supporting a specimen in the pathof said rays intermediate said source and said screen, electron focusingmeans for said rays fixedly disposed intermediate said source and saidspecimen supporting means, and adjustable electrical energizing meansoperable upon said focusing means for selectively varying the focus ofsaid rays from a point adjacent to said specimen and intermediate saidspecimen and said source to a point on said screen for selectivelyderiving an enlarged shadow image, an electron diffraction pattern, anda combined shadow image and diffraction pattern of said specimen on saidscreen.

9. Apparatus according to, ,claim 8 including 7 means for Avaryingsadfocus of said rays in diss crete steps.'- f- *10, Apparatus according toclaimv 8 including meansfof lvarying said focus of said rayscontinuously between predetermined focal limits.

v v JAMES HILLIER.

REFERENCES CITED Thefliowing references are of record in the file. pfthis patent:

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